WASHINGTON — All signs on Monday pointed to a frantic last-minute showdown that will determine whether or not the U.S. government stays open next week or shuts down as a result of Republican efforts to scuttle “Obamacare,” President Barack Obama’s health care law.

Heightening the tension, and the pressure on Republicans, the Pentagon issued a warning about the consequences of a shutdown, neither the first nor the last such announcement expected from federal agencies over the next few days.

Congressional authorization for the government to spend money runs out at the end of the fiscal year on Sept. 30, unless Congress passes a “continuing resolution” to keep the government running.

On Friday, Republicans in the House of Representatives passed and sent to the Senate a measure that would make continued funding of the government contingent on defunding Obamacare, which is designed to provide insurance coverage to millions of uninsured Americans.

The bill passed on a mostly partisan vote of 230-189.

The Democratic-controlled Senate is likely to strip out the Obamacare provision and send the funding bill back to the House, but it’s taking its time.

It could be as late as Sunday evening before it acts, giving the House less than 24 hours to sort out its many divisions and respond.

“Either pass a clean CR or shut down the federal government,” would be the Senate’s message to the House, Majority Leader Harry Reid said on Monday.

Any bill that defunds Obamacare “is dead on arrival in the Senate … We’re not going to bow to Tea Party anarchists,” Reid said, referring to the most ardent Obamacare opponents, who are increasingly influential in the Republican House.

Past shutdown showdowns — they have occurred in recent years whenever the government has been divided politically — have been accompanied by ever more serious-sounding warnings.

The Defense Department said on Monday it had been directed by the White House budget office to begin planning for a shutdown, an action it said would “put severe hardships on an already stressed workforce and is totally unnecessary.”

All military personnel would continue working regardless of the shutdown, Pentagon spokesman George Little said, but they might not be paid on time, depending on how long it lasted.

The government does not totally close down in such situations. There are many exceptions, including national security, emergency services, payments of Medicare health insurance and Social Security retirement benefits.

Public outcry

If Congress fails to meet its deadline, a shutdown could last a few hours or a few days but probably not longer, as it would inevitably be accompanied by a public outcry that has often in the past forced Congress’s hand.

A CNBC poll released on Monday showed that by a 59-19 percent margin respondents opposed linking defunding of Obamacare to a possible shutdown, or to a failure to raise the government’s borrowing authority, which is also expected to provoke a showdown by mid-October or early November.

Eighteen percent were undecided, according to the nationwide poll of 800 people conducted by Hart-McInturff. It had a margin of error of 3.4 percentage points.

The House includes 200 Democrats and 233 Republicans. A combined vote of roughly two dozen Republicans and almost all Democrats is thought to be the most likely route to avoiding a shutdown or to undoing one once it gets started.

The long Republican war against Obamacare is at the heart of the clashes ahead, just as important provisions of that law start taking effect on Oct. 1.

The only certainty is that when the dust settles, Obamacare will still be standing. The law was upheld by the Supreme Court last year.

House Republicans next week hope to unveil a second important bill — one to prevent the U.S. government from defaulting on its debt sometime in October or November.

The legislation would raise the country’s $16.7 trillion borrowing limit. Republicans again want to extract something from Democrats in return for the debt limit hike.

They have been talking about attaching yet another plan to the debt limit bill that would gut or delay portions of Obamacare, as well as forcing approval of the Keystone oil pipeline that would run from Canada to the Gulf of Mexico.

House Republicans say they also are looking at some tax provisions to possibly add onto the debt limit bill.

Obama has warned Republicans against loading up the debt limit bill with unrelated items, saying he will not negotiate on a measure that in effect pays the government’s outstanding bills.

Passage of a loaded-up debt limit bill in the House likely would trigger a rerun of the stop-gap spending fight that is consuming Washington now.

The battle over the shutdown and the debt ceiling is to some extent a repeat of fiscal showdowns that have taken up much of the past three years, since Republicans took control of the House in the 2010 elections.

With every one of them, the standing of Congress among Americans, never very high, has sunk even lower.